By way of background, there are three levels of signals produced for color television. At the highest quality level, the video signal produced by the television camera has red (R), green (G), blue (B) signal components. At the next level, referred to as the component level, the video signals are referred to as luminance (Y) and two chroma signals, a signal designated CR and a signal designated CB. At the lowest level, the video signals are in the composite format, which includes a luminance (Y) signal and two chroma signals, designated I and Q. These are the NTSC components, where the chroma signals are modulated and added to the luminance signal to form the NTSC composite signal. As used herein, the term digital video component signal refers to a digital representation of the color video signal at the component (Y, CR, CB) level. The term NTSC signal refers to the color video signal at the composite NTSC level in either the digital or analog format.
In a related, presently pending patent application, Ser. No. 424,829, filed on Oct. 19, 1989 in the names of Kevin Stec and Jerome D. Shields, there is described a method and apparatus for converting the digital video component signal into a representation of the digital video composite signal in a standard format known as D2 and for performing the reverse conversion. Those conversions both require rate conversion of the signals, since the digital video component signal uses a clock frequency (Sample Rate) of 13.5 MHz and the D2 signal format uses a clock frequency (Sample Rate) of 14.318 MHz. The D2 signal format is used in television studios for production and in distributing the signal between television studios and production facilities.
The digital video component signal is a better quality signal and can be manipulated more easily than the video composite signal. Thus, the digital video component signal is usually used by professionals for production work to do such things as inserting computer graphics, resizing a picture, overlapping, matting and creating special effects. Once the production work is completed, that signal format must be converted to the NTSC signal format if it is to be displayed via standard television receivers. The present invention provides direct conversion of the digital video component signal to the NTSC signal format at 13.5 MHz for use in television receivers and does not require rate conversion.
In general, conversion between the digital video component signal format and the NTSC signal requires the following steps:
1. Matrixing of the chrominance components of the digital video component signal to form the chrominance components of the NTSC signal in accordance with the following mathematical relationship: EQU I=(1.032) CR+(-0.476) CB EQU Q=(0.670) CR+(0.733) CB
2. Band limiting of the chroma components in separate low pass filters with different characteristics to produce, respectively, the two chroma component filter outputs, LPF.sub.I and LPF.sub.Q.
3. Modulation of the 3.58 MHz color carrier by the two chroma components filter outputs and adding the luminance component to form the digital NTSC signal in accordance with the following mathematical relationship: EQU Digital Component NTSC=Y+LPF.sub.I cos [(2 .pi.(3.58.times.10.sup.6 t+33/360)]+LPF.sub.Q Sin [(2.pi.(3.58.times.10.sup.6 t+33/360)]
4. Converting the digital NTSC siqnal into the analog NTSC signal using a digital-to analog conversion.
There have been systems in the prior art which convert the digital video composite signal into the analog NTSC signal. These systems have involved conversion of digital signals into analog signals at various intermediate stages in the processing and completing the conversion by further processing of the resultant analog signals. These prior art systems suffer from problems associated with analog processing of signals, such as circuitry whose response varies with temperature, the necessity for making frequency, phase, and level adjustments, and the relatively high cost of analog circuitry.
Accordingly, it is an object of the present invention to provide a system for direct conversion between the digital video composite signal and the analog NTSC signal which relies upon digital signal processing and which eliminates the analog circuitry previously required for such conversion. This direct digital conversion in accordance with the invention provides a better quality picture and repeatability of performance not found or achievable with the use of analog circuitry and processing found in the prior art.
It is another object of the invention to provide a digital filter for the chroma signals with enough taps to satisfy the performance criteria of the system. It is a further object of the invention to provide a single digital filter for both luminance and chroma signals. Another object of the invention is to provide a coefficient RAM which stores sets of coefficients for processing the luminance and chroma signals.